Katherine
I have gone through the 1851 census twice for this family at Langstone and they are not there, which is strange because Mary was baptised on the 6 April 1851 and the 1851 census was taken on 31 March only 6 days before.
I expect you are trying to find the birthplace of Elizabeth, my suggestion would be to purchase the birth certificate for Mary so you have the Mothers maiden name, then find the marriage which must be around 1851 or just before. then where ever they married may give you some idea of where they were in 1851.
Regarding Mary Watkins the 1851 census says she was born Langstone so she must have been born after 31 March. This is all of a tight squeeze.
There is always the possibilty they are missed from the Langstone census. Come back to me if you have any other info. I have the 1851 census for nearly every county in the area.
Mike